The invention relates to a fuel injection system for maintaining suitable operational conditions during passive operation (engine overrun, for example during downhill operation) and during the transition from passive operation to active operation. The fuel injection system includes a circuit which generates triggering pulses in proportion to the engine rpm and also a control multivibrator for generating fuel injection control pulses, the duration of which is determined substantially by the engine rpm and the aspirated air quantity. These control pulses are fed, after possible further correction, to electromagnetic fuel injection valves.
Already known are intermittently or continuously operating fuel injection systems which provide fuel in the required and desired ratio to the aspirated air quantity in accordance with the needs of the engine. In such systems, a large number of sensors determines the prevailing operational condition of the engine and generates injection control signals which are fed to the injection valves so that the fuel requirements are adapted precisely to the prevailing condition of the engine and to the desires of the operator. The primary variables used for metering out the fuel to an internal combustion engine in the known system are the rpm of the engine and the air quantity per stroke. Mainly from these two values, the injection system calculates the duration of the injection pulses. However, a particular operational state of the engine is difficult to determine with customary sensors and is also difficult to translate into injection control pulses of suitable duration. One of the unusual conditions is, for example, the situation when the operator of a vehicle which had been running at high engine rpm suddenly removes his foot from the gas pedal, permitting the throttle plate to return to its idling position. In that case, the fuel injection system receives sensor signals indicating relatively high engine rpm as well as substantial vacuum in the induction manifold but the throttle plate is in its zero position so that the engine operates as a passive prime mover, i.e., as if it were pushed, and the torque M supplied by the engine assumes negative values.
Even under such rapidly changing conditions and the onset of engine overrunning, the fuel injection system must be capable to provide a fuel mixture that does not detonate in the exhaust system even when the operator reaccelerates from that condition. Furthermore, the fuel mixture must be such as to maintain an acceptable exhaust gas composition in such operation.